It should be recalled that the use of conversion products curable (crosslinkable) by UV radiation or an electron beam (EB) such as adhesives, protective varnishes, lacquers, inks and paints, is widely used at the present time in printing and surface varnishing. This is because, compared with conventional products based on organic and aqueous solvents, these products have advantages from the technical standpoint (rapid crosslinking, little material shrinkage, quality of the end-product and easy cleaning of the printing plates) and from the environmental standpoint (100% solids content resins and lower energy consumption).
Since the crosslinking step has to be carried out on an industrial scale continuously, 24 hours a day, the chamber having one or more UV lamps is an open system. Consequently, the crosslinking mechanism that takes place in the zone irradiated by the UV lamp is carried out in the atmospheric air. This step is carried out in industrial plants with run speeds ranging from 10 to a few hundred m/min depending on the application.
Most products that crosslink by UV radiation are radical systems. The formulation contains, in addition to the base chemical constituents, such as a prepolymer, a reactive diluent and additives, a photoinitiator (PI). Under the action of UV, this photoinitiator generates free radicals (step a) that will initiate the radical polymerization reactions according to the various steps described in scheme 1 below. The radicals (R*) react with the reactive functional groups (M) of the prepolymer and of the diluent, and initiate the polymerization reaction (step b). Since the reactive functional groups are both contained in the prepolymer and the diluent, the propagation (step c) of the polymerization reaction develops in three dimensions. In this way, termination (step d) of the polymer chain results in a highly crosslinked polymer network (R(M)n).

At the present time, the industrial ultraviolet equipment operates in open system and these radical photopolymerization reactions take place in the atmospheric air. Now, all the radicals (R*, RM* and R(M)n*) involved in the crosslinking process are highly reactive with respect to oxygen in the air. These radicals react with the oxygen to form peroxides (RO2*) and hydroperoxides (ROOH), thus reducing the effectiveness of the radical photopolymerization reactions (see scheme 2 below). Oxygen interferes at various levels of the chemical mechanism described above, with the effect of reducing the quantity of free radicals (step a), preventing the initiation of the polymerization (step b) and prematurely terminating the formation of polymer chains (step d).
These phenomena occur with oxygen initially present in the formulation and with the atmospheric oxygen that diffuses during UV exposure through the film of the UV resin. Oxygen can thus retard or completely inhibit the radical polymerization reaction. The inhibiting effect of oxygen is all the more pronounced when the thickness of the UV resin layers is small.

The practical consequences of these phenomena are:                no polymerization of the UV-coating;        formation of short chains, and therefore a film of ink, adhesive or varnish of mediocre quality;        formation of quality-detracting labile oligomers (appearance, odor, health problems if food contacts with the substrate for example); and        formation of peroxides (RO2*) and hydroperoxides (RO2H) partly responsible for yellowing of the product.        
The importance of the atmosphere composition inside a chamber for the UV crosslinking of resins, and more particularly the absence of oxygen in the UV zone, is therefore well understood. Consequently, it is essential for certain applications to have equipment capable of considerably reducing the oxygen concentration inside a UV chamber, and more specifically in the zone where the radical photopolymerization reactions take place. This equipment should allow the step of curing the UV resins to be optimized.
A number of existing solutions for remedying the drawbacks associated with the presence of oxygen when crosslinking UV resins may be listed.
A first solution consists in increasing the intensity of the UV lamps so as to increase the production of free radicals (according to reaction (a), scheme 1). These radicals, produced in larger quantity, react with oxygen present in the reaction zone and reduce the oxygen concentration of the chamber and therefore the inhibiting effect of oxygen.
This solution, although easy to implement, results in a higher consumption of electricity and therefore a not insignificant additional energy cost since the power of the lamps used is usually about 20 kW. Moreover, increasing the intensity of the lamps will raise the temperature inside the chamber (reaction zone) and therefore runs the risk of thermally degrading the coating.
A second solution consists in introducing into the formation large quantities of photoinitiators and molecules (synergists), the role of which is to react, and therefore remove, the oxygen present in the reaction zone. Even though these products are increasingly effective, it is estimated that, in current formations, 80% of the photoinitiators and of the synergists react with oxygen, and therefore destroy it, while the remaining 20% are used to crosslink the UV resins.
However, these chemical substances constitute the most expensive part of the formation and, in addition, they may be harmful and their use may cause yellowing of the crosslinked resin and a very strong odor.
Finally, a third solution consists in removing the residual oxygen present in the reaction zone and in replacing this oxygen with an inert gas, such as nitrogen. This solution means that the chamber—an open system, where the resin crosslinking takes place—has to be modified and equipped with a device for operating in an inert controlled atmosphere. The UV crosslinking of resins in a controlled nitrogen atmosphere has many advantages since the absence of oxygen in the UV zone makes it possible to increase the crosslinking rate, to reduce the light intensity of the UV lamps or the number of UV lamps used, to reduce the quantity of photoinitiators and synergists introduced into the formulation, and to reduce the formation of by-products (such as peroxide and hydroperoxides), while still obtaining an end-product of very high quality.
Moreover, it should be pointed out that such working conditions in an inert atmosphere have the advantage of limiting the formation of ozone in the chamber.
Document WO 00/14468 for example has proposed equipment for operating with about 50 ppm of residual oxygen in the reaction zone, with speeds reaching several hundreds of meters per minute. This equipment is characterized by the presence of two gas injection units placed at the entry and exit of the UV chamber. Each of these units comprises two gas injection systems. The first injection system, placed at the ends of the chamber, has the function of preventing any air from entering the chamber, while the second injection system, placed inside the chamber, has the function of filling the chamber with nitrogen. The first injection system is a slot oriented in such a way that the stream of gas is directed toward the outside of the chamber. The second injection system is a tube possessing pores oriented so that the stream of gas is directed toward the inside of the chamber. The width of the slot and the orientation angles of the two injection systems can be modified and depend on the operating conditions.
However, the gas volumes needed for a low residual oxygen concentration for operation at the speeds used are very high (or even very considerable). As an example, at 200 m/min, the quantity of nitrogen must be 140 Sm3/h for a concentration of less than 50 ppm. In addition, the discharge of a large quantity of nitrogen to the outside of the UV chamber in the working zone requires an effective extraction system in order to avoid any risk of asphyxia by anoxia.
It may also be pointed out that the Applicant has proposed, in document WO 02/40738, equipment for controlling and managing the gases during operations requiring control of the atmosphere inside a chamber. The operations intended by that prior document were especially electrical-discharge surface treatments at atmospheric pressure in the presence of a gas mixture and in a controlled atmosphere, or else operations of the UV curing and EB curing type. According to this prior work, the recommended equipment comprises:                entry and exit devices adjacent the chamber in order to prevent air from entering the chamber and to prevent gaseous effluents exiting therefrom, respectively;        an extraction device comprising a line opening into the chamber; and        means for regulating the flow of gas extracted by said extraction device so as to maintain an approximately zero pressure difference between the inside of the chamber and the surrounding atmosphere.        
Each of the entry and exit devices typically consist (see FIG. 1 below; the reader may also refer to FIG. 2 of said document WO 02/40738) of three components positioned in series and seen in succession by the treated substrate, namely a channel, a gas injection slot and a “labyrinth”. The concept of a “labyrinth” is explained in detail in this prior document, and relates in fact to a system of open grooves facing the internal space (gap) of the entry (or exit) device in question (through which gap the substrate to be treated runs) and forming a labyrinth.
The channel, separated from the gas injection slot by a partition, is open facing the internal space of the entry or exit device in question.
The gas (nitrogen) injected through the slot allows the entrained air boundary layer on the surface of the film to be detached. This is because the labyrinth, by creating an overpressure zone (large pressure drop) in the direction in which the film runs, forces the nitrogen to flow toward the upstream, that is to say into the channel. This phenomenon is favored by a lower pressure drop in the channel. This turbulence in the channel creates a zone of slight underpressure on the surface of the film, which detaches the air boundary layer located at the surface of the film. The stream of nitrogen in the channel then becomes a laminar flow and forms a piston effect that opposes the stream of air, pushing it back. The combination of these three elements (channel, nitrogen knife, labyrinth) makes it possible, at the inlet, to prevent air from entering the chamber while minimizing the consumption of nitrogen. The same labyrinth seal placed at the outlet makes it possible to prevent the gaseous effluents from leaving the chamber.
This equipment proves to be remarkably effective since it allows a film surface treatment to be carried out in the presence of an oxygen concentration not exceeding 50 ppm with acceptable nitrogen volumes.
The use of this prior equipment for reducing the oxygen concentration during the crosslinking of coatings by UV radiation has of course been envisaged. However, it is clearly apparent that, for at least the following reasons, this equipment is not optimized for meeting this technical objective: firstly, the UV crosslinking method does not include a surface treatment and therefore does not require a nitrogen-based treatment gas to be injected into the chamber. But secondly, the absence of harmful gaseous effluents formed in the UV zone makes it unnecessary to use a central extraction system for removing them, which extraction system is, as a consequence, generally absent from such installations.
It is therefore apparent that substantial modifications of this prior equipment be recommended in order to meet this new technical problem.